1. Field of the Invention
The present invention relates to a clutch mechanism in a power driven screwdriver for driving and tightening screws.
2. Description of the Prior Art
U.S. Pat. No. 4,655,103 discloses a clutch mechanism in a power driven screwdriver. When a screw has been driven by a predetermined depth, the clutch mechanism of this patent interrupts transmission of rotation from a motor to a spindle so as to stop a tightening operation and to permit an idle rotation of the motor without producing undesirable clutch sounds.
The construction of the clutch mechanism of the above patent will now be explained with reference to FIG. 12 which corresponds to FIG. 2a of this patent.
A clutch mechanism 50 includes a first clutch element 51, a second clutch element 52 and an intermediate clutch element 53. The first clutch element 51 is rotatable relative to a spindle 54 but is fixed in position in an axial direction. The spindle 54 is movable in the axial direction relative to the first clutch element 51. The intermediate clutch element 52 is rotatably and axially movably supported by the spindle 54. The second clutch element 52 is fixed to the spindle 54.
A drive gear 51a is formed on an outer peripheral surface of the first clutch element 51 and is in engagement with a pinion motor (not shown). A cam 51b is formed on a lower portion of the first clutch element 51 and functions as clutch teeth. A cam 53a is formed on an upper portion of the intermediate clutch element 53 for engagement with the cam 51b. An engaging portion 53b is formed on a lower portion of the intermediate clutch element 53. An engaging portion 52a is formed on an upper portion of the second clutch element 52 for engagement with the portion 53b.
A compression coil spring (not shown) interposed between the first clutch element 51 and the intermediate clutch element 53, so that the intermediate clutch element 53 is biased in direction away from the first clutch element 51 or in a direction toward the second clutch element 52.
In operation, the first clutch element 51 and the intermediate clutch element 53 are in engagement with each other, and the intermediate clutch element 53 and the second clutch element 52 are also in engagement with each other until a forward end of a stopper sleeve 55 abuts on a work W into which a screw S is driven. The rotation of the motor is therefore transmitted to the spindle 54 via the clutch mechanism 50, so that the screw S is driven into the work W. Even after the forward end of the stopper sleeve 55 has abutted on the work, the spindle 54 is still rotated to further tighten the screw S, so that the intermediate clutch element 53 and the second clutch element 52 are moved downwardly together with the spindle 54 and that the intermediate clutch element 53 is moved away from the first clutch element 51 which is fixed in position in the axial direction. When the screw S has been driven by a predetermined depth, the intermediate clutch element 53 is disengaged from the first clutch element 51. Simultaneously therewith, the transmission of rotation to the intermediate clutch element 53 is interrupted, so that the rotation of the spindle 54 is stopped. The rotational torque applied to the intermediate clutch element 53 is thus interrupted, and at the same time therewith, the intermediate clutch element 53 is moved by the biasing force of the compression coil spring in a direction away from the first clutch element 51 or in a direction toward the second clutch element. An appropriate space is therefore formed between the first clutch element 51 and the intermediate clutch element 53, so that the first clutch element 51 idly silently rotates.
However, with this prior art clutch mechanism 50, the provision of the intermediate clutch element 53 is essential to attaining an idle rotation of the first clutch element 51. Therefore, the clutch mechanism 50 uses an increased number of parts and is heavy in weight. Additionally, since a space is required for interposition of the intermediate clutch member 53, the power driven screwdriver is long in size. Further, the cam 53a and the engaging portion 53b functioning as clutch teeth must be formed on the upper and lower portions of the intermediate clutch element 53, respectively. Therefore, the intermediate clutch element 53 must have a considerable durability, a wear resistance and a machining accuracy. The manufacturing costs of the clutch mechanism 50 having the intermediate clutch element 53 therefore increase.